It is well known to utilize a video display system in the analysis of x-ray photographs and other subjects. Typically, a television camera is employed to scan the x-ray photograph and produce video signals which are applied to a video display device, e.g., a television monitor, to reproduce the scanned photograph in a video display. Such systems advantageously allow adjustment of the brightness and contrast of the video display to more distinctly reveal the information depicted in the x-ray photograph. Normally, the adjustment features of the video system simultaneously affect the entire video display with the result that all parts of the display are subject to exactly the same level of adjustment. This type of total adjustment in brightness or contrast provides little or no assistance in achieving a more distinct and useful video display when analyzing areas of the x-ray photograph where the information is obscured by excessive brilliance or darkness. In addition, low contrast areas of the x-ray photograph are difficult to analyze by simple adjustment of brightness or contrast which may well serve to obscure the desired information. These problems are especially encountered in attempting to use x-ray photographs in the diagnoses of soft tissue maladies, e.g., breast cancer, or in detection of items such as plastic explosives in luggage examined by x-ray equipment at airports and other security areas.
In applicant's aforesaid co-pending application, a video display system is disclosed for concentrating signal components of interest in the low frequency range and utilizing a low-pass filter to eliminate high frequency signal components (noise) from the video signal. The filtered video signal is applied to a window amplifier to expand the visibility over a selectable range of signals. The system improves the visibility in the video display by adjusting the expansion factor and mid-point of the window amplifier to enhance the image contrast of that portion of the image of particular interest in the investigation. However, since the contrast adjustment achieved by applicant's visibility expander system affects the entire video display image, information in specific areas which cover a wide range of contrasts is often partially obscured by burn-out and/or complete darkness for the viewing surface; the same result occurring in other areas of the video display with high brightness levels making viewing difficult. The resulting burn-outs and darknesses tend to obscure the information contained in the x-ray photograph as well as making it difficult for the eye to adjust to the extremes in brightness of closely adjacent viewing regions. Similarly, areas of excessive blackness may appear on the video display which also obscure the desired information.
These areas of excessive brightness or blackness are extremely difficult to improve with conventional brightness and contrast adjustments. Moreover, even with applicant's improved visibility expander system, the adjustment of the window amplifier to operate on signals of predetermined amplitude in its high gain range may simultaneously serve to generate areas of excessive brightness and blackness in the video display which actually contain important information not visible on the video display. Thus, an improved video system which allows a great expansion of image contrast to permit differentiations between adjacent regions of slight contrast differences, such as are encountered in viewing soft tissue abnormalities while concurrently permitting control of excessive contrast in selected regions, is of great importance. This capability allows the important areas of an x-ray photograph to be thoroughly scrutinized.
Accordingly, it is a primary object of the present invention to provide a video display system which has the capability of concentrating on and improving the visibility of information in particular areas of the video display.
Another object of the invention is to provide a video diaplay system which is readily adjustable to eliminate areas of excessive brightness and darkness from the video display while permitting a great expansion of contrast in adjacent or selected other regions.
It is also an object of the invention to provide an improved video display system which is more effective in distinguishing detail in low contrast regions of the video display while concurrently permitting the reduction of excessive brightness or darkness in selected regions.
A further object of the invention is to provide a video display system in which the video signal is modified by a shading signal having signal components corresponding to linear and non-linear functions of the horizontal and vertical deflection signals to vary the intensity of the video display at selected regions selectable by the use of the sweep signals utilized to produce the position variable shading signals.
It is a further object of the invention to provide a video display system in which a shading circuit is utilized to modify the video signal by superimposing independently variable quadratic and linear signal components of the video sweep signals to selectively vary the intensity of the video display as a function of the region of the display.
Yet another object of the present invention is to provide a shading circuit for automatically controlling the intensity of the display.